Georg Bar

Effect of viscoelastic properties of polymers on the phase shift in tapping mode atomic force microscopy

On the basis of the frequency-sweep tapping mode atomic force microscopy (TMAFM) experiments for polystyrene (PS) and poly(dimethylsiloxane) (PDMS), the validity of the harmonic approximation for describing the phase shift of TMAFM was tested by evaluating the analytical expressions derived from the approximation. Our study shows that the phase shift is almost quantitatively described by the harmonic approximation. By analyzing the results of these experiments, it was also examined how the viscoelastic properties of a polymer affect the nature of the tip-sample interaction. The nature of the tip-sample interaction is strongly affected by the sharpness of the tip and by the elasticity, viscosity, and plasticity of a polymer sample. On a strongly glassy, brittle sample with a low impact strength, such as PS, the phase shift can be dominated by the attractive force with the use of a dull tip, while on a strongly elastic sample, such as PDMS, the phase shift can be dominated by the indentation force with the use of a sharp tip.

Description of phase imaging in tapping mode atomic force microscopy by harmonic approximation

Abstract Tapping mode atomic force microscopy measurements were performed for an elastomer, polydimethylsiloxane, to obtain amplitudes and phase angles as a function of driving frequency at various driving and set-point amplitudes. The amplitude curves support the harmonic approximation that the vibrational characteristics of a tapping cantilever are essentially harmonic. Based on this approximation, we derived simple analytical formulas describing the phase shift, the predictions of which agree reasonably well with experiment. Our work shows that the phase shift is influenced by the dissipation energy as well as the resonance frequency shift associated with the tip–sample interaction, and suggests a simple way of estimating the dissipation energy.

Novel approach for the preparation of metal colloid monolayers on modified surfaces

The morphology of Au colloid monolayers on modified silicon oxide surfaces (glass, silicon wafer, and ITO slides) has been studied. These monolayers were prepared by a new method described here. We also demonstrate several applications of this colloid monolayer surface. The preparation route creates a rough metal surface, the morphology of which can be easily controlled. The colloid monolayers are prepared in two steps: (1) modification of the substrates with starburst dendrimers; (2) noble metal colloid deposition onto the dendrimer layer. Au particles of approximately 28 nm in diameter were deposited onto the dendrimer‐modified surfaces. The spontaneous adsorption of the dendrimers to the silicon oxide surface was proven by x‐ray photoelectron spectroscopy, contact angle, and grazing angle Fourier transform infrared spectroscopy. The thickness of the dendrimer layer has been calculated to be in the range of d=14–25 A. The structure and properties of the resulting particle arrangement have been studied b...

Atomic scale surface studies of conductive organic compounds 3. Scanning tunneling microscopy studies of monocrystals of bis(ethylenedithio)tetrathiofulvalene with triiodide, ß-ET2I3

Abstract Scanning tunneling microscopy (STM) was applied to monocrystals of an organic superconductor, the charge transfer salt of bis(ethylenedithio)tetrathiofulvalene with triiodide, s-ET 2 I 3 , under ambient conditions. The obtained results reveal details of the charge distribution on the surfaces of differently shaped monocrystals. The periodicity of STM patterns obtained from the largest face of an elongated plate-like monocrystal is compatible with that of the crystallographic (001) plane. The STM patterns are therefore compared to drawings of corresponding computer-simulated models based on known crystallographic data. The imaged surface layer seems to consist of I 3 − anions. This interpretation agrees with the assumption that the central iodine atom of the anion has a higher charge density than the adjacent atoms. STM images of a surface of another (block-like) monocrystal display a pattern which is compared with the atomic arrangement in the (001) plane of the crystallographic structure. Well-separated linear patterns may be interpreted as a visualization of two-dimensional conductive layers parallel to the (001) plane.

High-Angle Annular Dark Field Scanning Transmission Electron Microscopy on Carbon-Based Functional Polymer Systems

Two purely carbon-based functional polymer systems were investigated by bright-field conventional transmission electron microscopy (CTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). For a carbon black (CB) filled polymer system, HAADF-STEM provides high contrast between the CB agglomerates and the polymer matrix so that details of the interface organization easily can be revealed and assignment of the CB phase is straightforward. For a second system, the functional polymer blend representing the photoactive layer of a polymer solar cell, details of its nanoscale organization could be observed that were not accessible with CTEM. By varying the camera length in HAADF-STEM imaging, the contrast can be enhanced between crystalline and amorphous compounds due to diffraction contrast so that nanoscale interconnections between domains are identified. In general, due to its incoherent imaging characteristics HAADF-STEM allows for reliable interpretation of the data obtained.

Synthesis and characterization of oxazoline-functional polymer particles by free radical nonaqueous dispersion polymerization

A novel oxazoline-functional methacrylate was prepared and employed as comonomer to produce nonaqueous dispersions of oxazoline-functional polymer particles. In nonaqueous free radical dispersion copolymerization of methylmethacrylate in the presence ofoxazoline-functional methacrylate, ethyleneglycoldimethacrylate crosslinking agent, AIBN initiator, and polystyrene-block-poly(ethene-alt-propene) dispersing agent, the average polymer particle size, varying between 100 and 500 nm, was controlled by the dispersing agent contents. According to titration with HCI0 4 all oxazoline groups regardless of their location at particle surface or bulk, were accessible. Glass transition temperature decreased from 120 to 0°C when oxazoline functional methacrylate was increased from 0 to 95 mol %. As imaged by atomic force microscopy incorporation of the new oxazoline-functional methacrylate improved film formation. Oxazoline-functional polymer particles were easy to redisperse in a variety of other diluents.

Microstructure of Banded Polymer Spherulites: Studies with Micro-Focus X-ray Diffraction

Micro-beam X-ray diffraction has been used to investigate the texture of banded spherulites of melt-crystallized poly(trimethylene terephthalate), PTT, formed in films of approximately 30 to 50 μm thickness. The WAXS micro-diffraction patterns show that at the local scale, the PTT texture is close to that of a single crystal. In agreement with previous studies using selected-area electron diffraction, it is shown that the crystal growth direction is parallel to the a-axis of the unit cell. When plotted as a function of the distance to the spherulite center, the intensity of different diffraction peaks reveals the same periodicity. This means that the lamellar twist is strictly uniform. The latter observation is more compatible with the model explaining the twist as a result of unbalanced surface stresses than that of isochiral giant screw dislocations. The main features of the experimental diffractograms can be understood using the numerical approach, which is developed in the approximation of a purely geometric broadening of X-ray reflections. In particular, the simulation can predict the sequence of appearance of different diffraction peaks and their shape on the 2D micro-diffraction patterns.

Interpretation of scanning tunneling and atomic force microscopy images of TCNQ salts

Abstract The surfaces of the TCNQ salts TTF-TCNQ, Qn(TCNQ) 2 , 4EP(TCNQ) 2 and TEA(TCNQ) 2 (TCNQ = tetracyanoquinodimethane, TTF = tetrathiafulvalene, Qn = quinolinium, 4EP = 4-ethyl- N -methylpyridinium, TEA = triethylammonium) were examined by scanning tunneling microscopy (STM) and atomic force (AFM). The STM and AFM images of these salts were interpreted on the basis of the ϱ ( r 0 , e f ) and ϱ ( r 0 ) plots of their molecular layers calculated with the extended Huckel tight-binding electronic structure method. The present work shows that a close interplay between experiment and theory is necessary for the rational surface analysis with STM and AFM.

Surface analysis of organic superconductors by scanning probe techniques: STM and AFM

Abstract Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used for surface characterization of organic superconductors — bis(ethylenedithio)tetrathiafulvalene (ET) salts with linear anions I - 3 , IBr - 2 , AuI - 2 and Cu(NCS) - 2 . The atomic-scale STM and AFM images correlate well with the surface molecular arrangement, simulated from the X-ray diffraction bulk data: two-dimensional image parameters are close to the crystallographic ones. However, several peculiarities of surface electronic and atomic structure have been revealed. For this purpose the comparison of STM and AFM images of the same surfaces was useful. The variations of atomic scale details in STM images, recorded at different tunneling conditions, are discussed.

Patterned self‐assembled monolayers of ferrocene and methyl terminated alkanethiols on gold: A combined electrochemical, scanning probe microscopy, and surface science study

We prepared patterned self‐assembled monolayers (SAMs) consisting of hexadecanethiol (16AT) and ferrocenyldodecanethiol (12FAT). The samples were characterized by scanning electron microscopy, lateral force microscopy (LFM), Auger electron spectroscopy, x‐ray photoelectron spectroscopy (XPS), electrochemistry and contact angle measurements. LFM images exhibit contrast even between surface regions of quite similar hydrophobicity. The 12FAT regions undergo irreversible chemical changes and become electrochemically inactive upon long exposure to the laboratory atmosphere. These chemical changes can be monitored by LFM, XPS, contact angle measurements and electrochemistry. The LFM images of the exposed and contaminated samples show a reversed frictional contrast relative to the LFM images of the fresh samples and to the LFM images of the exposed but ethanol‐rinsed sample. Based on these observations, the mechanism of the LFM image contrast is discussed and other driving forces, arising not only from differenc...